The present invention relates to an apparatus for the electrochemical surface treatment of substrates and more particularly to an electroplating rack.
The treatment of a wide variety of substrates by electroplating is generally known. This technique of depositing individual metals or even metal alloys on the surface of substrates is primarily used if the layer to be electrodeposited is intended to impart to the treated substrate certain properties which the substrate does not itself have or does not adequately have, such as, for example, electrical conductivity, luster, reflecting power, chemical resistance, etc.
Since individual metals or alloys may also be deposited successively on the surface of the substrate, it is possible to impart desired properties to the substrate in a very controlled manner; for example to achieve an appropriate corrosion resistance for the substrate.
Initially only substrates which had metallic properties themselves were subjected to the technique of surface treatment by electroplating. In the case of these substrates, it was possible to carry out the metallic deposition on the surface by electroplating apparatus directly and without difficulty by applying an electric field, the substrate simply being connected as cathode in the process.
After an ever increasing number of plastic fiber substrates were used in practice for a wide variety of applications, a surface treatment by electroplating also became standard in the case of these substrates. For this purpose, the electrically nonconducting plastic surfaces are first activated by depositing a catalytically active substance and then metallized by mechanical means. The electrically nonconducting plastic surfaces are therefore provided with a metallic coating which is subsequently reinforced by electroplating in a suitable manner with the same metal and/or even another metal.
The application of the above-mentioned technology to textile fabrics, non-woven materials, needle-bonded felts or open-pore foams opened up completely new fields of application for these materials. Starting from the materials mentioned, it has become possible, as a result of the preceding chemical deposition and subsequent electrodeposition of metal on the plastic surface, to combine the highly porous properties of these plastic products in an advantageous manner with metallic properties such as, for example, magnetism, screening capacity or electrical conductivity.
The surface treatment of metallized substrates by electroplating is in general carried out in a manner such that the substrate to be electroplated is mounted on an electroplating rack. The substrate has to be adequately electrically connected to the electroplating rack under these circumstances. This is done, for example, by suspending, clipping and/or clamping the substrate on the electroplating rack which is provided with an insulating plastic layer over its entire surface except at the contact points with the substrate. The electroplating rack supporting the substrates is introduced into the electroplating bath and the process of electroplating is carried out.
The electroplating racks used are generallY composed of an iron containing material, for example of a stainless steel. Firmly adhering plastics, for example polyvinyl chloride or polyethylene, in the form of a continuous layer or in the form of a plastic adhesive tape are used as an insulating layer. Layers of rubber or of wax are also known as insulating materials (W. Dettner and J. Elze "Handbuch der Galvanotechnik" ("Manual of Electroplating" (1963), Volume I, Part 1, pages 514 ff).
When such electroplating racks which are covered (except at the contact points with the substrate) are used, it has repeatedly been found that dendrite-like structures of the metal deposited during electroplating are formed while the electroplating process is being carried out and even grow through the insulation of the electroplating rack. The formation of these dendrites on the metallic parts of the rack can be explained by the fact that the plastic layer deposited on the rack has fine pores from the outset.
Alternatively, microcracks or microholes are produced in the plastic layer as a result of use of the rack (for example, due to sharp edges in the rack), with the result that the electroplating solution is able to penetrate to the metallic parts of the rack. The high current density in the electroplating bath and an inadequate electrical conductivity of the substrate to be electroplated occurs at the start of the electroplating process, particularly in the case of the treatment of premetallized plastic surfaces. Removal of the metal dendrites which have grown through the plastic insulation is very time-consuming and, in addition, associated with high cost. Renewal of the plastic insulation at the damaged points can also be carried out only with a high cost expenditure.
An object of the present invention is therefore to provide an apparatus for the electrochemical surface treatment of metallic substrates or metallized plastic substrates, in which the metallic rack has an impermeable and resistant insulating layer and consequently the above-cited adverse phenomena of the formation of dendrite-like structures on the metallic parent body are no longer able to occur during the electroplating process and consequently the electroplating rack remains serviceable for a longer service life.
This object and other objects are achieved, according to one embodiment of the present invention, in that an enamel layer is deposited on the metallic rack as an insulating layer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.